staging: comedi: adv_pci1710: rename private data member 'ai_et_MuxVal'

Rename thie CamelCase member of the private data. Add a comment in
the interrupt handler to clarify why the channel interval is updated
again.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>